It's done in adaptations
Obviously do an auto scan first and save it. We do this alot for our customers. That said we are not responsible for anything you do this is free advice only.
ENG248428-ENG248530-function_configuration_audio-channel_1_TT,not installed , change this to installed.
ENG248428-ENG248604-function_configuration_audio-channel_4_TT,not installed , change this to installed.
This activate left and rear speakers if you installing coaxials if it's components there's 2 other channels to turn on but I need to look at my notes.
ENG248428-ENG248818-function_configuration_audio-Signal_flow_topology, the value is 10 change this to 20. This turns on the fader.
Restart the head unit by either ticking the soft reset box before clicking the do it button for the fader or press and hold the power button for 30 seconds so the unit restarts but do this once all the Adaptation changed have been done.
Hope this helps. I got jumped on by some guys on another forum for giving out this info. But as a business I don't see the issue. If you have VCDS people will figure it out eventually.